The invention generally relates to both a method and a printer for printing an image onto a carrier from numerical image data which utilizes a single microprocessor both to modulate a thermal printhead and to implement all of the printing functions.
Thermal printers include a single row of thermal elements for printing a single line of an image onto a print carrier such as paper. Each thermal element is in reality an electrical resistance heater that fuses dye from a donor onto the print medium. When a particular thermal element is energized by a flow of electrical current therethrough, it generates heat which causes dye from a donor (such as a dye ribbon) to transfer onto the image carrier. The density or tone of the printed image is a function of the temperature of the thermal printing element and the time the element is heated.
In the prior art, the modulation of the thermal printing elements (i.e., the actuation and deactuation of each element to achieve the desired color tone) has been implemented by a high-speed, application specific integrated circuit (hereinafter referred to as an ASIC). The ASIC translates a series of binary numbers generated by the microprocessor into print commands for the thermal elements, while the microprocessor coordinates the other printing functions, such as paper and ribbon advances, etc. The applicant has observed that both the structural complexity and expense of manufacturing such thermal printers could be significantly reduced if the microprocessor could be used to modulate the printhead, thereby obviating the need for the ASIC. However, before the difficulties associated with such a design can be appreciated, some further background as to the structure and operation of such printers is necessary. In the printhead of such printers, the thermal printing elements are arranged in a row which may include anywhere from 512 to several thousand printing elements. Each of the thermal elements in the printhead is intermittently connected to a power source via switches in the form of NAND gates. The NAND gates either connect or disconnect their respective thermal elements to the power source in response to "0" or "1" data bits received from a latch circuit. Each of the latch circuits is in turn connected to a one-bit wide gate of a shift register which receives a string of image data from an ASIC. In operation, the shift register serially loads data bits from the stream of image data into the latch circuits through its gates in accordance with clock pulses supplied by the microprocessor. In a printer wherein the tone of each pixel printed by a thermal element is described by a six-bit binary number, 64 possible data bits in the form of "1s" and "0s" are admitted through each shift register gate for every line of a specific color printed by the thermal printhead, which in turn allows the printhead to generate 64 different tones of a color per pixel.
The ASIC generates each string of data bits by converting each of the six-bit binary numbers into a 64 bit data stream. It then serially loads the first data bit from each stream into the shift register for each of the 512 thermal elements, whereupon the shift register gates are simultaneously opened to send the first of 64 actuating "1s" or deactuating "0s" to the thermal elements. This procedure is repeated 63 more times, whereupon the printing of a single line of the image has been completed. The printer then advances the print medium a predetermined line increment, and the process is repeated until the complete image is rendered.
The design of such thermal printers could be simplified and their manufacturing costs reduced if the microprocessor which controls the other functions in the printer could somehow also function to modulate the printhead, thereby eliminating the ASIC. Additionally, the use of a microprocessor for modulation would confer a number of desirable engineering flexibilities to the operation of the printer, as the microprocessor could be programmed to accommodate different modulation schemes used in different printers (i.e., pulse width modulation versus pulse count modulation), as well as different modulation functions in order to improve print quality (i.e., front end or tail end loading of actuating data bits, etc.). However, despite the recent advances in the operating speed of commercially available microprocessors (such as RISC type processors), there are still no cost effective microprocessors presently available that are capable of performing all of the computations necessary to expand the image data binary numbers and collate selected bits therefrom into the shift register while still performing the incremental paper advances and other functions necessary for the printer to operate.
Clearly, there is a need for a novel printer and method of operation which would allow a single microprocessor to effectively modulate the thermal printhead while still performing the other functions incident to a printing operation. Ideally, such a printer and method should be simple and inexpensive to construct and implement so that all of the advantages of structural simplification and cost reduction could be realized.